This invention relates to methods of decontaminating secondary (i.e., not virgin or primary) cellulosic fiber or fiber sources during repulping. More particularly, the present invention relates to methods of substantially removing contaminants such as waxes, adhesives, and the like from secondary fiber, dispersing them in the aqueous repulping medium, and removing the repulping medium from the pulp.
The present invention provides nearly complete removal of contaminants from the secondary fiber, and sufficiently permanent dispersion of them to prevent their deposition within the newly formed sheets or on production equipment. These contaminants, commonly referred to as "stickies", encompass a wide variety of substantially thermoplastic organic resinous water-insoluble contaminants such as waxes, hot melt adhesives, pressure sensitive adhesives, vinyl acetate-type coatings, SBR resins, latex adhesives, and others. Inks and the above substantially organic resinous water-insoluble contaminants form a broad category of deposits commonly referred to as "stickies and tackies".
These contaminants which can remain the secondary fiber after repulping or can later reagglomerate in the paper production waters can create a variety of series problems for the paper making industry, particularly in the liner board and corrugated industries. The contaminants typically form deposits on all types of paper making equipment, including the press roll and wet end of corrugating machines, press section calendars, dryer surfaces, wires, press felts, and the like. As a result, production time is lost and machine efficiency reduced due to sheet breaks at the breaker stacks, wrinkles at the rewinder, limited machine speeds due to wire filling, down time for deposit cleanup, and the like. Further these deposits typically require significant amounts of solvents and manpower for their removal, further contributing to their undesirability. Additionally, the stickies and tackies generally contaminate the paper sheets themselves, forming deposits which result in sheet defects such as holes, deposits, specks, and low brightness. In corrugated medium mills, frequently the product comprising secondary fibers will have poor absorbency because of the incorporated contaminants. This is a problem because absorbency is required for proper glue penetration of the corrugated products.
Many types of paper making utilize repulped secondary fibers. The paper products comprising such fibers include tissue, toweling, specialties grades, liner board, corrugated medium, boxboard and so on. Particularly the corrugated medium mills and liner board mills have turned to secondary fiber obtained from pre-used corrugated as a low cost fiber substitute. However, the ratio of secondary fiber to virgin fiber which can be utilized has been limited, due to the problems associated with stickies deposits in the system and sheets.
Until recently, the major form of control for stickies and tackies has been mechanical. Pressure screens, reverse cleaning systems and flotation loops have been used to remove these contaminants from secondary fiber stock flows in processes such as tissue, toweling, corrugating board, and secondary liner board manufacture. Chemical approaches to the problem have incorporated talc, polymers, dispersants, and so on, but with little success. In the manufacture of corrugating and liner board, steam and asphalt dispersion units are sometimes used in an attempt to control these deposits. However, these machines are typically expensive to purchase and even more expensive to operate on a continual basis. Further, even where contaminants may appear to have been dispersed by known methods of controlling contaminants, these methods do not appear to prevent the contaminants from re-agglomerating, for example when the repulped paper fibers are being used to form a paper product. When the contaminants reagglomerate, the above-described problems of deposition will typically occur. Frequently the paper-making process is a continuous process from the repulping of secondary fiber until the new product is dried. Typically the production waters are redistributed or re-used through the system. Consequently, dispersed contaminants which are not sufficiently dispersed or which do not remain dispersed ultimately can cause problems as the process waters are recirculated through the system.
Accordingly, use of repulped secondary fiber originally containing contaminants such as wax and adhesives continues to be a significant source of lost production and expense due to the tendency of these contaminants to form deposits within the paper making system and on the sheets.
The present invention provides substantially reduced (if not eliminated) deposition of objectionable contaminants on processing machinery and in the sheets themselves where secondary fiber is utilized in a paper making process. Secondary fiber processed according to this invention will generally allow for increased production, less down time for deposit removal, faster machine speeds due to elimination of web breaks caused by sticky felt and rolls, improved drainage, faster drying, and the like.